diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index a09eaa437946..45bed7df12f3 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -482,7 +482,7 @@ async def try_backfill(domains: List[str]) -> bool:
 
         processing_end_time = self.clock.time_msec()
         backfill_processing_before_timer.observe(
-            (processing_start_time - processing_end_time) / 1000
+            (processing_end_time - processing_start_time) / 1000
         )
 
         success = await try_backfill(likely_domains)
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 13bc9482c535..fec4b003971a 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -674,7 +674,7 @@ async def on_GET(
         room_member_count = await make_deferred_yieldable(room_member_count_deferred)
         messsages_response_timer.labels(
             room_size=_RoomSize.from_member_count(room_member_count)
-        ).observe((processing_start_time - processing_end_time) / 1000)
+        ).observe((processing_end_time - processing_start_time) / 1000)
 
         return 200, msgs